Known hybrid vehicle drive systems coupling an internal combustion engine and an alternative power source such as an electric motor or a hydraulic motor require means of linking the alternative power source and the internal combustion engine to the drive wheels. Each configuration may have a preferred configuration, but such configuration can vary with vehicle use and application.
One known electric hybrid system, of the general type described in U.S. Pat. No. 7,463,962, has an electric motor coupled to a transmission input shaft. This configuration was well suited for use with a transmission having a countershaft powerflow configuration. However, by placing the motor in line with the engine and transmission, increasing the size of the electric motor requires making many significant changes to the associated driveline components and mounting features at great expense. It is desired to have an arrangement which permits greater flexibility in changing the size of the electric motor or electric motor/generator, and which is also well suited to a countershaft transmission. Countershaft transmissions are also known as mechanical or manual transmissions, in part because countershaft transmissions have been shifted manually by the vehicle operator. Automated countershaft transmissions are known as automated mechanical transmissions or AMTs.
One hydraulic hybrid system, known as a hydraulic launch assist (HLA) system, has been adapted for commercial vehicles and increases fuel economy and acceleration compared to vehicles not so equipped, particularly when used in application having frequent starting and stopping and low-speed operation, such as city buses and refuse collection trucks. However, HLA systems are typically used in combination with conventional automatic transmissions employing a torque converter to communicate driving torque from an engine to the transmission. The torque converter facilitates starting the vehicle from a stopped condition without the need to gradually engage a clutch, and the torque converter also provides torque multiplication when there is a significant speed ratio across the torque converter. At low speed operation, the torque converter losses are a much more significant portion of the power from the engine. A countershaft transmission, and more particularly an automated mechanical transmission (AMT) equipped with a plate clutch for transmitting torque, is significantly more efficient at low speed and start-stop operation than a torque converter transmission, and weighs less than a torque converter transmission. However, typical dry friction clutch plates or driven discs wear out undesirably quickly under such operating conditions. Additionally, the rate of acceleration when starting from a stop is typically less for an AMT equipped vehicle than a torque converter/automatic transmission equipped vehicle in part because of the torque multiplication benefit conferred by a torque converter. It is desired to have an arrangement which permits the coupling of an HLA system with a countershaft transmission and reduces the driven disc wear concern and improves the acceleration of the system over the acceleration provided by an AMT with a dry friction clutch.